designing-tests

What this skill does

# Designing Tests

## Test Implementation Workflow

Copy this checklist and track progress:

```
Test Implementation Progress:
- [ ] Step 1: Identify what to test
- [ ] Step 2: Select appropriate test type
- [ ] Step 3: Write tests following templates
- [ ] Step 4: Run tests and verify passing
- [ ] Step 5: Check coverage meets targets
- [ ] Step 6: Fix any failing tests
```

## Testing Pyramid

Apply the testing pyramid for balanced coverage:

```
        /\
       /  \     E2E Tests (10%)
      /----\    - Critical user journeys
     /      \   - Slow but comprehensive
    /--------\  Integration Tests (20%)
   /          \ - Component interactions
  /------------\ - API contracts
 /              \ Unit Tests (70%)
/________________\ - Fast, isolated
                   - Business logic focus
```

## Framework Selection

### JavaScript/TypeScript
| Type | Recommended | Alternative |
|------|-------------|-------------|
| Unit | Vitest | Jest |
| Integration | Vitest + MSW | Jest + SuperTest |
| E2E | Playwright | Cypress |
| Component | Testing Library | Enzyme |

### Python
| Type | Recommended | Alternative |
|------|-------------|-------------|
| Unit | pytest | unittest |
| Integration | pytest + httpx | pytest + requests |
| E2E | Playwright | Selenium |
| API | pytest + FastAPI TestClient | - |

### Go
| Type | Recommended |
|------|-------------|
| Unit | testing + testify |
| Integration | testing + httptest |
| E2E | testing + chromedp |

## Test Structure Templates

### Unit Test
```javascript
describe('[Unit] ComponentName', () => {
  describe('methodName', () => {
    it('should [expected behavior] when [condition]', () => {
      // Arrange
      const input = createTestInput();

      // Act
      const result = methodName(input);

      // Assert
      expect(result).toEqual(expectedOutput);
    });

    it('should throw error when [invalid condition]', () => {
      expect(() => methodName(invalidInput)).toThrow(ExpectedError);
    });
  });
});
```

### Integration Test
```javascript
describe('[Integration] API /users', () => {
  beforeAll(async () => {
    await setupTestDatabase();
  });

  afterAll(async () => {
    await teardownTestDatabase();
  });

  it('should create user and return 201', async () => {
    const response = await request(app)
      .post('/users')
      .send({ name: 'Test', email: '[email protected]' });

    expect(response.status).toBe(201);
    expect(response.body.id).toBeDefined();
  });
});
```

### E2E Test
```javascript
describe('[E2E] User Registration Flow', () => {
  it('should complete registration successfully', async ({ page }) => {
    await page.goto('/register');

    await page.fill('[data-testid="email"]', '[email protected]');
    await page.fill('[data-testid="password"]', 'SecurePass123!');
    await page.click('[data-testid="submit"]');

    await expect(page.locator('.welcome-message')).toBeVisible();
    await expect(page).toHaveURL('/dashboard');
  });
});
```

## Coverage Strategy

### What to Cover
- ✅ Business logic (100%)
- ✅ Edge cases and error handling (90%+)
- ✅ API contracts (100%)
- ✅ Critical user paths (E2E)
- ⚠️ UI components (snapshot + interaction)
- ❌ Third-party library internals
- ❌ Simple getters/setters

### Coverage Thresholds
```json
{
  "coverageThreshold": {
    "global": {
      "branches": 80,
      "functions": 80,
      "lines": 80,
      "statements": 80
    },
    "src/core/": {
      "branches": 95,
      "functions": 95
    }
  }
}
```

## Test Data Management

### Factories/Builders
```javascript
// factories/user.js
export const userFactory = (overrides = {}) => ({
  id: faker.string.uuid(),
  name: faker.person.fullName(),
  email: faker.internet.email(),
  createdAt: new Date(),
  ...overrides,
});

// Usage
const admin = userFactory({ role: 'admin' });
```

### Fixtures
```javascript
// fixtures/users.json
{
  "validUser": { "name": "Test", "email": "[email protected]" },
  "invalidUser": { "name": "", "email": "invalid" }
}
```

## Mocking Strategy

### When to Mock
- ✅ External APIs and services
- ✅ Database in unit tests
- ✅ Time/Date for determinism
- ✅ Random values
- ❌ Internal modules (usually)
- ❌ The code under test

### Mock Examples
```javascript
// API mocking with MSW
import { http, HttpResponse } from 'msw';

export const handlers = [
  http.get('/api/users', () => {
    return HttpResponse.json([
      { id: 1, name: 'John' },
    ]);
  }),
];

// Time mocking
vi.useFakeTimers();
vi.setSystemTime(new Date('2024-01-01'));
```

## Test Validation Loop

After writing tests, run this validation:

```
Test Validation:
- [ ] All tests pass: `npm test`
- [ ] Coverage meets thresholds: `npm test -- --coverage`
- [ ] No flaky tests (run multiple times)
- [ ] Tests are independent (order doesn't matter)
- [ ] Test names clearly describe behavior
```

If any tests fail, fix them before proceeding. If coverage is below target, add more tests for uncovered code paths.

```bash
# Run tests
npm test

# Run with coverage
npm test -- --coverage

# Run specific test file
npm test -- path/to/test.spec.ts

# Run in watch mode during development
npm test -- --watch
```